Session V28: Focus Session: Carbon Nanotubes: Chemistry

Resonance Raman spectroscopy of length fractionated single-walled carbon nanotubes

In many potential applications of single-walled carbon nanotubes (SWNTs), the difficulty of separating nanotubes by their structural properties, \textit{e.g.}, length and chirality, remains an impediment to their widespread implementation. Our studies include HiPco, CoMoCat, and arc-discharge SWNTs wrapped with 30-mer 5'-GT(GT)$_{13}$-3' single-stranded DNA and dispersed in solution. These samples display an exceptionally low degree of SWNT bundling and clustering, in the limit of nanodispersion.\footnote{J. A. Fagan \textit{et al.}, J. Phys. Chem. B (in press).} Size-exclusion chromatography collects length fractions ranging in size from $< 100\,$nm to $\approx 400\,$nm. Multi-angle light scattering, AFM, and TEM characterize the length distribution of each fraction. We measure resonance Raman spectroscopy (RRS) over a wide range of laser excitation wavelengths for vibrational modes including the radial breathing mode (RBM) and higher order graphite modes. All modes exhibit a monotonic increase of Raman scattering intensity with increasing nanotube length. We discuss these results in terms of an optical scattering model. Furthermore, we hybridize these length separated fractions with complimentary DNA sequences functionalized with nanoparticles to study the effects of DNA-wrapping on SWNT properties.   

Modeling of the SWNT-DNA complexes in the water solution

It is known that the single-wall nanotubes (SWNTs) may form a hybrid with a single-stranded DNA having a regular helical structure of the DNA wrap around the SWNT cylinder. Such DNA wrapping creates a periodic potential at the NT surface, which results in developing a specific modulation of the NT bands. Numerical self-consistent modeling of these effects requires knowledge of the polarization of the environment. We have shown that the result is very sensitive to what extent the exterior water (and ions in the solution) are polarized to screen the potential of the DNA. Both the NT screening and the response of the environment are important to include self-consistently to obtain quantitative results. We present the Monte-Carlo simulation of the interaction of a NT, a DNA and a solvent and provide heuristic physics interpretation of the results. We show that the NT screening is different from what one expects for a metal or insulator material due to non-local Coulomb correlations. An effective dielectric screening of the water exterior is extracted from the simulations.   

Electronic structure of single-walled carbon nanotubes inside helical DNA wraps

Single stranded DNA can helically wrap a single-walled carbon nanotube (SWNT) leading to changes in electronic structure, which is the subject of our study. Other charged polymers may produce band gap modulation similar to that observed for DNA-SWNT complexes. For these hybrids we assume a regular helical wrap, the potential of which breaks the symmetry of the pristine SWNT. Band structure changes are modeled quantum mechanically using the tight binding method together with self-consistent electrostatics. Gap modulation and band structure symmetry-lowering effects may result in variation of the optical spectra, especially for (slightly forbidden) transverse optical transitions. The effect of environmental screening of charges is investigated. Self-consistent electrostatic calculations yield cohesion energy between a charged, regular wrap and a SWNT of the order of tenths of eV per DNA base [1]. [1] Snyder, S. E., and Rotkin, S. V., Polarization Component of Cohesion Energy in Single-Wall Carbon Nanotube-DNA Complexes, JETP Letters 84, 348 (2006).   

Aminoacid Functionalization and Raman Characterization of DWNT

Carbon nanotubes are difficult to manipulate because of their aggregation and low reactivity. For this reason many types of functionalization have been obtained and, usually, large functionalizations are needed. However, in particular considering single wall carbon nanotubes (SWNT), a large functionalization modifies their electronic properties because it introduces a large number of defects states. To overcome this problem we have considered double wall carbon nanotubes (DWNT) which can be considered as SWNT protected by an external carbon nanotube. We have performed an oxidation and a functionalization of DWNT covalently linking charged aminoacids. From the Raman characterization of the functionalized nanotubes we find that the external nanotubes have been modified by the functionalization but not the internal ones. We think that this is an interesting approach to obtain carbon nanotubes which are easy to manipulate but with electronic properties, in this case of the internal nanotube, which are preserved.   

Directed Linking of Carbon Nanotubes with CdSe Quantum Dots and Au Nanoparticles

As circuit miniaturization continues, the demand for smaller and more efficient component parts has increased. Metallic single-walled carbon nanotubes (SWNTs) are the ideal nanometer-scale wire, as they can withstand current densities up to 2 to 3 orders of magnitude higher than copper currently used in electronic chips. These conductive nanotubes can therefore be utilized as ``nano-electrodes'' to efficiently electrically contact another nanoscale object, such as a single semiconductor quantum dot (QD) or metallic nanoparticle (NP), thus creating macroscopic integrated systems based on nanometer-scale components. Although NPs have been previously attached to NTs, the attachment scheme was uncontrolled; direct and defined attachment of NPs to SWNTs remains elusive. We have designed a strategy for directed assembly of fabricated QD--SWNT devices. NTs were grown across patterned catalyst islands on a silicon wafer followed by electrode placement. After cutting the NTs, the resulting carboxylic group moieties found at the cut NT edges were used to covalently attach CdSe QDs or Au NPs. Electrostatic force microscopy (EFM) and transport measurements were used to monitor NT conductivity before and after cutting, as well as after NP attachment. The photoelectrical transport properties of a typical hybrid QD--SWNT device will be discussed.   

Size Selective Interaction of Single Wall Carbon Nanotubes with Collagen

One of the big challenges in using single-wall carbon nanotubes (SWNTs) in nanotube{\-}electronics at the present time is to produce SWNT's of specific diameters. Unfortunately, it is almost impossible to achieve this by existing synthesis procedures. All these produce SWNT's with a mixture of diameters and chiralities and, therefore, different electrical properties such as semiconducting and metallic. Here, we propose a method of functionalization that selects SWNTs of a single specific diameter from a mixture of tubes. We have shown that denaturation of collagen type-I solution in the presence of sodium dodecyl sulphate (SDS) and SWNT's leads to wrapping of carbon nanotubes of a specific diameter by collagen peptides, which are soluble in water. Separation is achieved by centrifugation of the solution at 10,000 RPM and taking the supernatant, which is rich in nanotubes having one specific diameter.   

Optical modulation of single walled carbon nanotubes

Recent advances in the spectroscopy of single walled carbon nanotubes have significantly enhanced our ability to understand and control their surface chemistry, both covalently and non-covalently. Our work has focused on modulating the optical properties of semiconducting single walled carbon nanotubes as near infrared photoluminescent sensors for chemical analysis. Molecular detection using near-infrared light between 0.9 and 1.3 eV has important biomedical applications because of greater tissue penetration and reduced auto-fluorescent background in thick tissue or whole-blood media. In one system, the transition of DNA secondary structure modulates the dielectric environment of the single-walled carbon nanotube (SWNT) around which it is adsorbed. The SWNT band-gap fluorescence undergoes a red shift when an encapsulating 30-nucleotide oligomer is exposed to counter ions that screen the charged backbone. We demonstrate the detection of the mercuric ions in whole blood, tissue, and from within living mammalian cells using this technology. Similar results are obtained for DNA hybridization and the detection of single nucleotide polymorphism. We also report the synthesis and successful testing of near-infrared $\beta$-D-glucose sensors2 that utilize a different mechanism: a photoluminescence modulation via charge transfer. The results demonstrate new opportunities for nanoparticle optical sensors that operate in strongly absorbing media of relevance to medicine or biology.   

Targeted Damage of Carbon Nanotubes

One possible pathway to fabricating a narrow strip of graphene would be to cut open a carbon nanotube. To that end, we present a preliminary procedure for selectively damaging carbon nanotubes. Nanotubes of diameter 3-10 nm were grown from iron nanoparticles and electrical contacts were lithographically applied to make transistor devices several microns long. To selectively damage the nanotubes a region was opened in a blanket of photoresist over the tube. A thin aluminum oxide layer (2-3 nm) was deposited at an angle in the exposed region to partially protect the nanotube. The nanotubes were then briefly exposed to an oxygen plasma. After the plasma etch and removal of the photoresist and aluminum oxide, a fraction of the nanotubes no longer conduct and others have higher resistance. For some of the nanotubes with increased resistance there is a height difference between the damaged and undamaged sections and scanned gate microscopy shows enhanced sensitivity in the etched region.   

{\em Ab Initio} Study of Crosslinking of Functionalized Carbon Nanotubes

We investigate the mechanism of covalent crosslinking between carbon nanotubes functionalized with thiocarboxylic and dithiocarboxylic esters. The structures of interconnected nanotubes are modeled in the framework of density functional theory combined with the pseudopotential approximation. Our calculations reveal an important role of surface defects in the formation of chemical bonds connecting nanotubes to each other. The strength and stability of intertube bonds increases in the vicinity of defect sites. The computed binding energies and potential energy profiles of linked nanotubes are found to be sensitive to the choice of the exchange-correlation functional used within the density functional formalism. This sensitivity could be explained by a nonuniform distribution of the electronic charge density near defect sites. Our results imply that the use of gradient-corrected functionals is essential for accurate theoretical modeling of functionalized carbon nanotubes and nanotube-based composites.   

Deformation of $sp^2$ graphitic nanostructure by irradiation with highly charged ion

I will discuss possibility of structural deformation of graphitic $sp^2$ network by irradiation of highly charged ions. Meguro {\it et al.}, [Appl. Phys. Lett..{\bf 79}, 3866 (2001)] reported indication of nano-diamond formation on graphite surface when it is irradiated by Ar$^{+8}$ ions with incident kinetic energy of 400 eV. The nano-diamond structure was suggested from STS and IR spectrum taken after irradiation showing energy gap of 6 eV and C-C stretching with the frequency of 1360 cm$^{-1}$. This structural change was considered to be induced by injection of holes from highly charged ion (Ar$^{+8}$) while role the kinetic energy of Ar ion was thought to be marginal. Electron-ion dynamics simulation combined to the time-dependent density functional theory [O. Sugino, Y. Miyamoto. Phys Rev B{\bf 59}, 2579 (1999), {\it ibid}, Phys. Rev. B{\bf 66}, 088901(E) (2002)] has been performed to examine the mechanisms of the structural change. This simulation has found that the role of incident kinetic energy of Ar$^{+8}$ and subsequent cooling mechanisms also play crucial role in determining the structural change. More details will be presented and discussed in my talk.   

Metallic carbon nanotubes destruction using Laser Irradiation

We demonstrated that, using laser irradiation in air, metallic single-walled carbon nanotubes (SWNTs) in carbon nanotube thin film can be preferentially destroyed to their semiconducting counterparts if SWNTs are not heavily bundled. Although all metallic SWNTs were not destroyed using the lasers with an excitation wavelength of 514.5nm and 632.8nm due to a large distribution of SWNTs diameter, it is clear that if SWNTs with a small distribution of diameter can be produced, it should be possible to destroy all of the metallic SWNTs using one or two lasers. [Huang et al. J.Phys.Chem.B, 2006, 110, 7316-20. and 4686-90.]   

First-principles characterization of carbon nanotubes functionalized with [2+1] cycloadditions

First-principles calculations predict that [2+1] cycloadditions of carbenes or nitrenes on single-wall carbon nanotubes can induce bond cleaving between adjacent sidewall carbons, recovering in the process the $sp^2$ hybridization of the pristine tubes \footnote{Y.-S. Lee and N. Marzari, Phys. Rev. Lett. 97, 116801 (2006)}. Electrical conductance is strongly affected by the local bonding environment, and the $sp^2$ re-hybridization induced by cycloadditions restores the conductance of the pristine tubes even in the presence of significant chemical or structural disorder. Phonon dispersions, Born effective charges, and polarizabilities of functionalized carbon nanotubes have been also studied, to provide a link between the local bonding structure and experimental Raman and infrared spectra.   

First Principles Calculations for Destructive Deformation of Carbon Nanotubes by Oxygen Molecule

It is well-known that carbon nanotube devices are significantly sensitive to ambient gas molecules such as oxygen molecules. Especially chemisorbed oxygen molecules have possibility to destruct carbon network and even destroy the devices. Then oxidation of carbon nanotubes is crucial for nano device application. On the other hand, the oxidation is very useful method to cut the cap structures or purify nanotubes by burning the defective nanotubes. Furthermore, some experimental results suggest that the chirality dependent oxidation of nanotubes. To understand the properties of oxidation, we focus on an initial process where the C=C bond is broken after the cyclo-addition of oxygen molecule on nanotube surface. The DFT-LDA calculations show that the diameter dependence and also the chirality dependence of reaction barriers to break the C=C bond. We will also discuss the effect of hole doping which would correspond to the oxidation process in solution.